Oncidium is a very large genus that contains about 650 species that are widespread in Latin America and the Caribbean.

The plants are mostly epiphytes, but there are also a few lithophytes and terrestrials.

It is a very complex genus, with many changes overtime and with more changes expected. I believe you will be better of if you check the internet for information. I could not possibly mention everything here.

The plants can grow from small, only a few cm tall, to very tall, up to 5 meters. The pseudopulbs differ widely in size from tiny to very large. The inflorescences can be very large and pendulous, the numerous flowers, 100 or more, are showy, some are fragrant, mostly yellow colored or brown, rarely white or pink, and long lasting.

Oncidium species and hybrids require plenty of light and mild temperatures in winter, roughly like cattleyas. Some species can be grown mounted but most growers grow them in pots. Oncidiums are propagated by division.

There are numerous factsheets online, for example from AOS.

It is always wise to read as much as you can and improve your knowledge, before you buy any plants.